Michiko B. Inoue

Potentiometric and 1H NMR studies of complexation of Al3+ with (−)-epigallocatechin gallate, a major active constituent of green tea

The acid dissociation of (-)-epigallocatechin gallate (abbreviated as egcg) and its complexation with Al(3+) were studied by potentiometric titrations, and were compared with those of (-)-epicatechin (ec) and (-)-epigallocatechin (egc). In Al(3+)-ec and Al(3+)-egc reaction systems, [Al(LH(-2))](+), [Al(LH(-2))(OH)](0), and [Al(LH(-2))(2)](-) are formed, as reported for Al(3+)-catechin (c). Reactions between Al(3+) and egcg at pH <4.1 yield AlLH(-2) and AlLH(-3) species. The 1H NMR studies have shown that two hydroxyl groups of the gallate (D) ring are deprotonated and coordinated to an Al(3+) ion in [Al(egcgH(-2))](+). The AlLH(-3) species of egcg is supposed to be formulated as [Al(egcgH(-3))](0) in which one hydroxyl group of the pyrogallol (B) ring and two hydroxyl groups of the D ring are deprotonated; an Al(3+) ion is coordinated to two oxygen atoms of the D ring and one oxygen atom from the B ring of the neighboring chelate molecule, resulting in the formation of a polymeric structure. In the Al(3+) complex of egcg, the gallate group forms major coordinate bonds and results in solution properties that are different from those of ec, egc and c which have no gallate group.

Complexation of electroconducting polypyrrole with copper

Abstract A copper complex of polypyrrole is obtained when the polymer perchlorate is treated with a Cu(II)-containing alkaline solution. The X-ray photoelectron spectrum of the resulting polymer material shows that the copper atoms are coordinated to pyrrole nitrogen. The ESR signal of copper ions and that of the pyrrole rings are observed independently of each other; there is no appreciable magnetic interaction between the two paramagnetic species. The room-temperature conductivity is 4 × 10−4 S cm− and the temperature dependence shows semiconductive behavior with an activation energy of 0.11 eV. The degradation of conducting polypyrrole in aqueous solution is suppressed by the presence of copper: the stabilization is attributed to Cuue5f8N bond formation.

Binuclear structure in the ammonium salt of gadolinium(III) diethylenetriaminepentaacetate, (NH4)4[Gd2(DTPA)2]·6H2O

Abstract A single crystal X-ray analysis of the ammonium salt of Gd 3+ diethylenetriaminepentaacetate has shown that the compound has a binuclear structure in contrast to the corresponding sodium salt that has a mononuclear structure. Five oxygen atoms and three nitrogen atoms from a ligand molecule construct a distorted square antiprism around a Gd 3+ ion. One of the square planes is capped by an oxygen atom from the adjacent metal chelate molecule, resulting in the formation of a binuclear metal chelate. The coordination polyhedron including this oxygen atom is described as a tricapped trigonal prism. The binuclear structure is the result of well-developed hydrogen bonds that involve the ammonium ions.

1H NMR and protonation constants of new metal-chelating macrocycles, oxocyclopolyamines with pendant carboxymethyl groups, and the stabilities of their Mg2+ and Ca2+ complexes

Potentiometry and 1H NMR have been used to study the protonation of a new series of 12−, 13−, 15−, 16− and 17-membered metal-chelating macrocycles with two or three pendant carboxymethyl groups. In the 12− and 13-membered macrocycles, ethylenediaminetetraacetate and ethylenediamine or propanediamine units are linked by two amide groups; in the 15−, 16−, and 17-membered macrocycles, diethylenetriaminepentaacetate and diamine units are linked by two amide groups. The protonation constants Kn (n = 1, 2, 3) of these new macrocyclic ligands are significantly smaller than the corresponding values for tetraaza and triaza macrocycles such as 1,4,7,10-tetraazacyclododecane-N,N′,N″,N″′-tetraacetic acid (DOTA) and 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA). The protonation sites have been identified by measuring the 1H NMR chemical shifts at different pD values. Protonation corresponding to K1 occurs on the amine nitrogen atoms. In the successive protonations, protons are distributed on the amine nitrogen atoms and the carboxylate oxygen atoms so as to minimize electrostatic repulsion. The low basicities of the amine nitrogen atoms in these new macrocycles are attributable to the introduction of amide groups into the ring system. As a consequence of the low basicities of the macrocycles, the formation constants of their Mg2+ and Ca2+ chelates are much lower than the corresponding values of EDTA. The ratio of the two formation constants, Kf(Ca)Kf(Mg), varies in an unpredictable manner with the size of the macrolytic ligand.

Syntheses of new 15-membered and 16-membered macrocyclic ligands with three pendant acetato groups and the structures of the gadolinium(III) complexes

Abstract A condensation of diethylenetriaminepentaacetic dianhydride with ethylenediamine gave a 15-membered macrocyclic ligand with three pendant acetato groups, (15-dtpa-en)H3ue5fbC10H18N5O2(CH2CO2H)3; a l6-membered analogue, (l6-dtpa-pn)H3 ue5fbC11H20N5O2(CH2CO2H)3, was obtained by the use of propanediamine instead of ethylenediamine. The structures of their gadolinium(III) complexes, Gd2(15-dtpa-en)2·16H2O and Gd(16-dtpa-pn)·4H2O, were determined by X-ray analyses. Gd2(15-dtpa-en)2·16H2O crystallized in the orthorhombic space group Pbca with: a=18.205(1), b=18.930(1) and c=15.609(1) A. Two Gd(III) ions are located between two ligand molecules, forming a binuclear metal chelate molecule with a center of inversion. The coordination geometry around a metal ion is described as a distorted tricapped trigonal prism that consists of nine coordinated atoms. Gd(l6- dtpa-pn)·4H2O crystallized in the monoclinic space group P21/c with: a =8.246(2), b = 14.995(3), c= 19.367(4) A and β = 90.258(2)°. In this compound, a water molecule and a single ligand molecule are coordinated to a Gd(III) ion, forming a mononuclear chelate with a tricapped trigonal prism. The structural differences between the two Gd(III) complexes are a result of the differences in the favorable conformations assumed by the two macrocyclic ligands.

X-ray structures and fluorescence spectra of binuclear Zn2+ and Cd2+ complexes of an amide-based naphthalenophane

Abstract The formation of binuclear Zn2+ and Cd2+ complexes with an amide-based chelating naphthalenophane has been confirmed by X-ray crystal analyses: the complexes are formulated as [(H2O)MLM(OH2)]0 (M=Zn or Cd), and the naphthalenophane, LH4, is 2,9,22,29-tetraoxo-4,7,24,27-tetrakis(carboxymethyl)-1,4,7,10,21,24,27,30-octaaza[10.10](1,5)naphthalenophane. The Zn2+ complex has a six-coordination geometry and the Cd2+ complex has a seven-coordination geometry. The naphthyl groups are deformed from the planar structure by metal complexation in both complexes. Fluorescence from the uncoordinated ligand is weakened by protonation. The coordination of Zn2+ enhances the fluorescence whereas the coordination of Cd2+ weakens the emission: the intensity ratio, F(L):F(Zn2L):F(Cd2L)=1:12:0.2 at pH 10. The Zn2+ complex exhibits a large change in the fluorescence intensity with pH, as a result of interconversion between [(H2O)ZnLZn(OH2)]0 and [(HO)ZnLZn(OH)]2−; the emission of the latter formed at higher pH is 20 times stronger than that of the former formed at lower pH. Structural changes that occur in the chelating units upon protonation or metal complexation propagate to the fluorescent units through the amide groups that link the two functional units. This propagation results in the fluorescence properties characteristic of the amide-based naphthalenophane.

Structural and spectroscopic studies of binuclear Cu2+ and Co2+ complexes with an amide-based naphthalenophane

Abstract Binuclear Cu2+ and Co2+ complexes with a chelating naphthalenophane were characterized by single-crystal X-ray analyses, electronic absorption spectroscopy and luminescence spectroscopy: the naphthalenophane is 2,9,22,29-tetraoxo-4,7,24,27-tetrakis(carboxymethyl)-1,4,7,10,21,24,27,30-octaaza[10.10](1,5)naphthalenophane (abbreviated as LH4). The Cu2+ complex crystallized as [Cu2L]0 from acidic solution and [Cu2(LH−4)]4− from basic solution: the coordination geometry around each metal ion in [Cu2L]0 is a square pyramid with an amide oxygen atom, two amino nitrogen atoms and two carboxylate oxygen atoms; the amide nitrogen atoms in [Cu2(LH−4)]4− are deprotonated and construct a square planar coordination geometry together with amino nitrogen atoms around each metal ion. The formation of the two structures is due to a change in the coordination linkage of the amide groups. [Cu2(LH−4)]4− shows strong metal–ligand charge transfer bands caused by the coordination of the amide nitrogen atoms that are directly bonded to the naphthyl groups. In the Co2+ complex, [Co2L(H2O)2]0, each metal ion has a seven-coordination geometry; the emission and excitation bands in the luminescence spectra showed a red shift upon metal complexation. In all metal complexes studied, the naphthyl groups are distorted from the planar structure, as a result of metal complexation that causes contraction of the macrocyclic rings.

High steric constraints and molecular distortion in methyl-substituted amide-based paracyclophanes and the binuclear Cu2+ complexes: X-ray structures, NMR and absorption spectra

Abstract Chelating paracyclophanes that are sterically constrained to a great extent have been synthesized and characterized by X-ray crystallography and NMR spectroscopy: the macrocycles studied are 2,9,18,25-tetraoxo-4,7,20,23-tetrakis(carboxymethyl)-1,4,7,10,17,20,23,26-octaaza[10.10]paracyclophane, abbreviated as ( L pd)H 4 , and its 2,5-dimethyl- p -phenylene and tetramethyl- p -phenylene derivatives, abbreviated as ( L dmpd)H 4 and ( L tmpd)H 4 , respectively. Steric interaction between tetramethylphenylene and amide groups in the tetramethyl derivative defines the conformation of the macrocyclic cavity, and causes unusual spectroscopic and chemical properties including the extreme line-broadening of 1 H NMR signals and the low basicity of amino nitrogen; such properties are not observed for the other macrocycles, in which steric interaction between phenylene and amide groups is less effective. The complexation of the highly strained ligand ( L tmpd)H 4 with Cu 2+ ions has been studied by X-ray crystallography and solution electronic spectroscopy. The macrocycle forms a binuclear complex of [Cu 2 (LH −4 )] 4− type in which four amide nitrogen atoms are deprotonated and each metal ion is coordinated to two amide nitrogen atoms and two amino nitrogen atoms. In the binuclear chelate molecule, the severe contraction of the macrocyclic ring forces the phenylene groups distorted to a boat form, due to the steric effect of the tetramethyl substituents. As a result, the metal–ligand charge-transfer interaction in the binuclear complex differs from that in the mononuclear chelate of the same macrocycle.

Solution 1H NMR and X-ray crystal structures of Ca2+, Zn2+ and Cd2+ complexes with 12− and 13-membered macrocycles, dioxotetraazacycloalkanediacetic acids

Abstract Structures in aqueous solutions of the complexes of Ca2+, Zn2+ and Cd2+ with 2,9-dioxo-1,4,7,I0-tetraaza-4,7-cyclododecanediacetic acid, abbreviated as (12edtaen)H2, and 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotridecanediacetic acid, (13edtapn)H2, were studied by 1H NMR spectroscopy at different pD. The formation constants of the Cd2+ complexes were determined by potentiometric titrations and compared with those of the Ca2+ and Zn2+ complexes. X-Ray crystal analyses were carried out on Ca2+ and Cd2+ complexes with (13edtapn)2− and the free ligand (12edtaen)H2, The ligand (12edtaen)H2, (C12H20N4O6) crystallized in the orthorhombic space group Pbcnwith a = 14.478(2) A , b = 9.605(1) A , c = 9.75I(1) A , and Z= 4 . The molecule has a C2 axis parallel to the molecular plane of the macrocyclic ring. The Cue5f8-N bond of the amide group has a partial double bond character and increases the rigidity of the ring framework. The Ca(13edtapn) complex, [Ca2(C13H20N4O6)2(H2O)4]·15H2O, crystallized in the triclinic space group P 1 with a = I0.969(2) A , b = 11.399(2) A , c = 10.931(2) A , α = 94.278(2)°, β = 94.009(2)°, γ = 111.971(2)° and Z= 1 . The coordination geometry around a Ca2+ ion is a distorted square antiprism formed by two carboxylate oxygen atoms, two amine nitrogen atoms and one amide oxygen atom from a ligand molecule, a carboxylate oxygen atom from a neighboring chelate molecule, and two oxygen atoms from water molecules. One of the carboxylate oxygen atoms bridges two Ca2+ ions, leading to the formation of a centrosymmetric binuclear structure. The Cd(13edtapn) complex, [Cd(C13H2N4O6)(H2O)]2·9H2O, crystallized in the orthorhombic space group Pccn with a = 16.260(2) A , b = 17.627(2) A , c = 15.251(2) A , and Z = 4 . A, distorted trigonal prism is formed around a Cd2+ ion by two carboxylate oxygen atoms, two amine nitrogen atoms, one amide oxygen atom and a water oxygen atom. The other amide oxygen atom is weakly coordinated to the metal ion with a Cdue5f8O distance of 2.857 A. The 1H NMR spectra of [Cd(13edtapn)]0 and [Zn(13edtapn)]0 in aqueous solutions show that two carbonyl oxygen atoms in the amide groups are coordinated to a central metal ion resulting in the formation of a seven-coordination geometry. The NMR spectra of the (12edtaen)2− complexes indicate that two carbonyl oxygen atoms in the amide groups are alternately coordinated to a central metal ion in the complexes in solution. The alternation rate in [Cd(12edtaen)]0 is higher than the NMR frequency, but in [Zn(12edtaen)]0 the alternation rate is close to the NMR frequency. The intramolecular exchange of the coordination sites decreases the lifetimes of the M–N bonds in these (12edtaen)2−complexes. The stabilities of the [ML]0 complexes with (12edtaen)H2, are almost identical with those of the corresponding metal complexes with (13edtapn)H2, The dynamic properties of the metal chelates are, however, different, due to the rigidity of the macrocyclic ring framework.

Charge-transfer complexes of benzylviologen (1,1′-dibenzyl-4,4′-bipyridinium(2+) cation) with cyanocuprate(I) and the structure of (BzV)3Cu9(CN)15·H2O

Reactions between 1,1′-dibenzyl-4,4′-bipyridinium(2+) (benzylviologen, BzV) chloride and cyanocuprates(I) gave two charge-transfer complexes having different colors: dark brown (BzV)3Cu9(CN)15·H2O and light brown (BzV)Cu(CN)3·2H2O. An X-ray crystal analysis of the former compound showed that nine crystallographically nonequivalent Cu atoms form three kinds of triad ue5f8Cuue5f8(CN)ue5f8Cuue5f8 screws, which are linked by CN groups resulting in a unique three-dimensional network structure. Three of the nine Cu atoms have distorted tetrahedral (td) coordination geometries while the others have triangular plane (tp) geometries. Each screw consists of a (-tp-td-tp-)n array. There are three crystallographically nonequivalent viologen molecules. Certain CuCN moieties are located above a viologen ring or by the side of a viologen ring, with close interatomic contacts. These close contacts are characteristic of the charge-transfer complex and are responsible for the deep color of the complex.